1. Field of Invention
The present invention relates to an angle detection signal processing apparatus for processing an angle detection signal of a resolver to finding angle data.
2. Description of the Related Art
In a variety of mechanical devices, determining the positional information of rotary shafts and moving parts is a fundamental function. For example, in a motor, in order to impart the optimum torque to a rotor, it is necessary to control the rotating magnetic field to the optimum field in accordance with the position of the rotor. For example, for automobiles, highly reliable, low cost angle sensors are demanded in the motors and power steering of hybrid cars. Further, there are manifold applications of such angle sensors, for example, bulldozers and other construction machines, various machine tools, production facilities, and further aerospace systems.
An extremely large variety of methods have been devised for detecting and controlling angles. For example, for the easiest control of angles, a stepping motor is used. This uses the number of pulses generated accompanied with rotation as information on the angle. However, the method using a stepping motor cannot detect slippage by the stepping motor per se when slippage occurs during the rotation, so cannot be said to be high in reliability.
For angle control with a high reliability, some sort of angle sensor is generally used. As a representative angle sensor, for example a sensor using a Hall-effect sensor to magnetically detect the relative positions of a magnetization pattern and a Hall-effect sensor and another sensor detecting the angle by an optical method such as an optical encoder are known. However, these are not sufficient for applications where an extremely high reliability is required. The use of a Hall-effect sensor cannot be said to be rugged against heat and vibration, while the optical method is susceptible to fouling by oil etc. and in addition requires a light source, therefore it suffers from the disadvantage of reliability.
At the present time, as a rugged angle sensor having the highest reliability, an angle sensor utilizing electromagnetic induction is known. This angle sensor is referred to as a “resolver” and has a structure resembling that of the motor in principle as shown in FIG. 29.
A coil 52 wound around a rotor 51 is excited by an excitation signal VE having a frequency of ωo. Two coils 54 and 55 are arranged in a stator 53 at right angles. When the rotor 51 rotates by exactly an angle θ(t) about a shaft, the following signals VI and VQ are induced in the coils 54 and 55 as in the following equations:VI=VO·Cos ωot·Cos θ(t)  (1)VQ=VO·Cos ωot·Sin θ(t)  (2)
An angle detection signal processing apparatus detects the angle θ(t) by this signal. Particularly, a signal processing apparatus outputting the angle in the form of digital data is called a “resolver-digital (R-D) converter”. A resolver basically has only a rotor, a stator, and a coil made of magnetic bodies, therefore, it has the features of being rugged and resistant to environmental effects such as dirt or temperature. For this reason, the resolver is the angle detection system most suitable for automobiles, bulldozers and other construction machines, machine tools, production facilities, and further aerospace systems and other applications where a high reliability is required.
For actual resolvers, a variety of structures have been proposed. FIG. 29 shows a basic structure. With this, a rotation brush is required for supplying an excitation current to the rotor. In order to eliminate that, a structure using a rotary converter or a newer structure in which the rotor is not given a coil, the rotor is excited by the coil of the stator, and a change in magnetic flux due to a change of the clearance between the rotor and the stator is sensed by the stator has been proposed. A resolver outputting the signals represented by equation (1) and equation (2) will be referred to as a “one-phase excitation two-phase output type” in the following description.
As explained above, among the variety of structures, when focusing on the signals obtained from the resolver as output, as shown in equation (1) and equation (2), a one-phase excitation two-phase output type resolver having a shaft rotation angle θ(t) and outputting a signal obtained by modulating the excitation signal by a cosine function Cos θ(t) and a sine function Sin θ(t) is most generally used.
On the other hand, when considering this from the ease of signal processing, a resolver outputting a product Cos θot×Cos θ(t) of cosine functions and a product Sin θot×Sin θ(t) of sine functions is better. However, the resolver outputting such signals must provide two independent rotors 56 and 57 and stators 58 and 59 on the same axis as shown in FIG. 30. Such a method is referred to as a “two-phase excitation two-phase output type”. The signal processing becomes very easy. However, it suffers from a large disadvantage in the structure, for example two independent pairs of rotors and stators become necessary, the mechanical structure is complex, and the structure becomes very thick, so this is used rarely.
The reason why the signal processing of the two-phase excitation two-phase output type becomes easy will be explained in brief next. The signals VI and VQ output from the two-phase excitation two-phase output type resolver may be represented by the following equations:
                              V          I                =                              Cos            ⁢                                                  ⁢                          ω              o                        ⁢                          t              ·              Cos                        ⁢                                                  ⁢                          θ                              (                t                )                                              =                                    1              /              2                        ⁢                          {                                                Cos                  ⁡                                      (                                                                                            ω                          o                                                ⁢                        t                                            +                                              θ                                                  (                          t                          )                                                                                      )                                                  +                                  Cos                  ⁡                                      (                                                                                            ω                          o                                                ⁢                        t                                            -                                              θ                                                  (                          t                          )                                                                                      )                                                              }                                                          (        3        )                                          V          Q                =                              Sin            ⁢                                                  ⁢                          ω              o                        ⁢                          t              ·              Sin                        ⁢                                                  ⁢                          θ                              (                t                )                                              =                                    1              /              2                        ⁢                          {                                                -                                      Cos                    ⁡                                          (                                                                                                    ω                            o                                                    ⁢                          t                                                +                                                  θ                                                      (                            t                            )                                                                                              )                                                                      +                                  Cos                  ⁡                                      (                                                                                            ω                          o                                                ⁢                        t                                            -                                              θ                                                  (                          t                          )                                                                                      )                                                              }                                                          (        4        )            
From the above equations, a cosine signal Cos(ωot+θ(t)) and Cos(ωot−θ(t)) are easily obtained by obtaining the difference and sum as shown below:VP=VI−VQ=Cos(ωot+θ(t))  (5)VN=VI+VQ=Cos(ωot−θ(t))  (6)
When the signals can be converted in this way, for example, by measuring the time difference at the zero cross point of the two signals, the angle θ(t) can be very easily found. FIG. 31 is a block diagram expressing that signal processing. First, by subtraction and addition of the signals VI and VQ, the signals VP and VN are found. Next, through a comparator, the zero cross of the signals VP and VN is found. Then, for example if calculating the rising edges thereof by a differentiation circuit and counting the number of clock pulses between the rising edges of the signals VP and VN by a counter, this becomes proportional to the found angle θ(t). Accordingly, from the count of this counter, an output obtained by digital conversion of the angle θ(t) can be extracted.
Next, an explanation will be given of an R-D converter widely used in a one-phase excitation two-phase output type resolver. FIG. 32 shows an example of the configuration thereof. For example, in order to obtain angle data of 12 bits, a read only memory (ROM) for storing a sine signal and a cosine signal having at least 11 bits of resolution, desirably 12 bits of resolution, is prepared, and a sine signal Sin φ(t) and a cosine signal Cos φ(t) are generated with respect to any angle φ(t). They are converted to analog signals at a D/A converter (DAC). In order to make the angle φ(t) correspond to the angle θ(t) to be found, the signal VI output from the resolver is multiplied by the sine signal Sin φ(t) and, at the same time, the signal VQ is multiplied by the cosine signal Cos φ(t). Then, the former is inverted and added to the latter, whereby the signal V1 shown in the following equation is generated.
                                                                        V                ⁢                                                                  ⁢                1                            =                            ⁢                                                                    -                    Cos                                    ⁢                                                                          ⁢                                      ω                    o                                    ⁢                                      t                    ·                    Cos                                    ⁢                                                                          ⁢                                                            θ                                              (                        t                        )                                                              ·                    Sin                                    ⁢                                                                          ⁢                                      φ                                          (                      t                      )                                                                      +                                  Cos                  ⁢                                                                          ⁢                                      ω                    o                                    ⁢                                      t                    ·                    Sin                                    ⁢                                                                          ⁢                                                            θ                                              (                        t                        )                                                              ·                    Cos                                    ⁢                                                                          ⁢                                      φ                                          (                      t                      )                                                                                                                                              =                            ⁢                              Cos                ⁢                                                                  ⁢                                  ω                  o                                ⁢                t                ⁢                                  {                                      Sin                    ⁡                                          (                                                                        θ                                                      (                            t                            )                                                                          -                                                  φ                                                      (                            t                            )                                                                                              )                                                        }                                                                                        (        7        )            
Further, the signal V1 of equation (7) is multiplied with the cosine signal Cos ωot for synchronization wave detection. By this, the component of the sine signal Sin{θ(t)−φ(t)} is extracted.
                                                                        V                ⁢                                                                  ⁢                2                            =                            ⁢                              V                ⁢                                                                  ⁢                                  1                  ·                  Cos                                ⁢                                                                  ⁢                                  ω                  o                                ⁢                t                                                                                        =                            ⁢                                                Cos                  2                                ⁢                                  ω                  o                                ⁢                t                ⁢                                  {                                      Sin                    ⁡                                          (                                                                        θ                                                      (                            t                            )                                                                          -                                                  φ                                                      (                            t                            )                                                                                              )                                                        }                                                                                                        =                            ⁢                                                                                          (                                              1                        +                                                  Cos                          ⁢                                                                                                          ⁢                          2                          ⁢                                                      ω                            o                                                    ⁢                          t                                                                    )                                        /                    2                                    ⁢                                      {                                          Sin                      ⁡                                              (                                                                              θ                                                          (                              t                              )                                                                                -                                                      φ                                                          (                              t                              )                                                                                                      )                                                              }                                                  ⇒                                                      (                                          1                      /                      2                                        )                                    ⁢                                      Sin                    ⁡                                          (                                                                        θ                                                      (                            t                            )                                                                          -                                                  φ                                                      (                            t                            )                                                                                              )                                                                                                                              (        8        )            
The cosine term Cos 2ωot in equation (8) has a high frequency, therefore is attenuated in a loop filter and only the low frequency term at the end of equation is extracted. The output of this loop filter is input to a bipolar voltage controlled oscillator (VCO). The bipolar VCO, as shown in FIG. 33, generates a pulse signal having a frequency proportional to an absolute value of the input signal and a polarity signal for judging the polarity of the input. An up/down counter performs an up count in accordance with the pulse signal of the bipolar VCO when the polarity signal is positive, while performs a down count when the polarity signal is negative. As a result, the count of the up/down counter becomes the digital data of the angle φ(t) per se.
The digital data of the obtained angle φ(t) are converted to the digital data of the sine signal Sin φ(t) and the cosine signal Cos φ(t) by the sine/cosine ROM and are converted to analog signals at the D/A converter. The loop filter has an integration characteristic and has an infinitely large DC gain, so the normal value of the input must be zero for the finite output. Accordingly, the angle φ(t) changes so as to follow the angle θ(t).
For reference, see Japanese Unexamined Patent Publication (Kokai) No. 11-83544.
The angle detection method using the zero cross point of signals as shown in FIG. 31 in a two-phase excitation two-phase output type resolver has the following disadvantages.
FIG. 34 is a graph showing an example of signal waveforms of portions in the circuit shown in FIG. 31. In the example of FIG. 34, the count of the clock pulse CP is started from a time t1 when the zero cross point of the signal VP occurs. The count ends at a time t2 when the zero cross point of the signal VN occurs. This count reflects the angle θ(t) and can be used as the digital value of the angle as it is. One count is extracted at the time t2.
In the circuit shown in FIG. 31, when the angle θ(t) quickly changes, it cannot be strictly defined at which point of time the digital value of angle obtained at the time t2 is detected as the data. This is because the zero cross point of the signal VP indicates the state of the signal VP at the time t1, and the zero cross point of the signal VN indicates the state of the signal VN at the time t2, therefore the angle θ(t) of the phase difference between the two can be only defined as the angle around the time t1 and t2 at most. Further, the output of the data of the angle at the time t2 means that a delay clearly occurs until the angle is detected and the data is obtained. In addition, data can be obtained only once for each cycle of the excitation frequency, so the continuous change of the angle cannot be grasped. Due to such a disadvantage, an R-D converter finding an angle by a zero cross point is not suited to, for example, an application where it is necessary to find the angle of a shaft rotating at a high speed in real time.
Another disadvantage of the method using the zero cross point of the signals is that this method lacks a high tolerance against an external noise. This is because when even slight noise enters near the zero cross point, the time of the zero cross point fluctuates.
On the other hand, the R-D converter shown in FIG. 32 is characterized in that that the output is obtained in real time. The up/down counter constantly holds the data of the angle φ(t). There exists a delay due to the angle φ(t) following the angle θ(t), but usually the delay is sufficiently short with respect to the mechanical movement of the angle θ(t). Further, this system compares the entire waveforms, therefore even if part has external noise superimposed on it, it is not susceptible to it as in zero cross detection.
However, the R-D converter shown in FIG. 32 has the disadvantages that the processing is complex and the circuit scale is large and as a result the power consumption is large and the cost is high. Namely, a complex bipolar VCO or up/down counter is needed. Further, in order to obtain a 12 bit resolution, a large capacity sine/cosine ROM having a resolution of at least 11 bits, desirably 12 bits and precision, and a D/A converter having a high resolution are required.
FIG. 35 is a graph for explaining the resolution necessary for the sine/cosine ROM and the D/A converter. In order to decompose an angle 2π by 12 bits, when considering the maximum inclination of the sine wave, a resolution of 1/π of 12 bits is necessary. Namely, when 2π is decomposed to 212 the maximum value of the step of the output becomes “1/(212/π)”. Accordingly, 10 bits are slightly insufficient, and 11 bits are necessary. When considering other error factors, 12 bits are desirable if desiring to give some extra margin. If simply provided by a table of the ROM, a considerably large sized memory is necessary. There is also the technique of reducing the amount of memory used by interpolation etc. In any case, this requires both an advanced large sized analog circuit and digital circuit, consumes large electric power, and is therefore expensive.
A bipolar VCO must start oscillating from zero frequency, therefore is difficult to realize as a circuit. In addition, a dead zone easily occurs near zero frequency, so there is the disadvantage that phase lock loop control becomes unstable.
An analog multiplier circuit is an element restricting the system performance. As the analog multiplier circuit, the circuit shown in FIG. 36 called a “Gilbert type multiplier circuit” is used for general purposes. This circuit is an analog function circuit widely used for example as a mixer of a wireless communication circuit (frequency mixing circuit). This circuit adeptly utilizes the characteristics of a bipolar transistor so as to be able to operate up to a high frequency with a very simple circuit. However, the absolute precision does not reach the required level for a function circuit for high precision signal processing such as an R-D converter requiring a precision of for example 12 bits or more. The dynamic range of the input of this circuit is for example about 20 mVp-p. The input conversion offset voltage with respect to that is typically about 1 to 2 mV. Accordingly, as the absolute precision, about 10% is just the guaranteed range. There is also the method of enlarging the dynamic range by an emitter feedback resistor or other means, but the offset voltage also increases, therefore the relative precision is not enhanced that much. Even when using trimming or another method together for a circuit solution, it is very difficult to guarantee an absolute precision of 1%. Accordingly, a precision of about 8 bits can be achieved, but this is far from 12 bits. For this reason, in the high precision R-D converter, it was necessary to use a multiplication type D/A converter etc. in place of a Gilbert type multiplier circuit to avoid the restrictions on precision of the multiplier circuit. Accordingly, the means for realizing the analog multiplier circuit becomes a factor inducing an increase of the power consumption and an increase of cost in order to achieve a high precision.